How to Build a Box Cooker
- South America, Peru
Learn how to build your own box cooker using Brian's step-by-step instructions.
Let's walk through the process of building a simple solar cooker. The blueprint I use can be found on the website solarcooking.org, where they call it the "minimum solar box cooker."
The box cooker is all about directing, trapping, and maintaining heat. It works a lot like a greenhouse. We use the top flap of the cooker to direct solar rays through the glass lid and into the oven. There, the aluminum foil acts as an insulator and helps to slowly raise the temperature of the oven. As more and more rays come into the oven, the temperature continues to rise until it gets to the boiling point of water, about 100 degrees Celsius. Everything about the oven is designed to either direct the rays and heat or to maintain the heat that we've achieved. As we go step by step through the construction process, we'll see how each aspect of the solar oven works. First of all, let' see what we'll need to make our solar oven.
Most of the materials are pretty self-explanatory but I'll go over a few of them and give some tips and tricks. You'll need:
Two cardboard boxes. I suggest that you use an inner box that is at least 15 inch x 15 inches, but bigger is better. The outer box should be larger than the small box all around, but it doesn't matter how much bigger, as long as there is a half-inch or more of airspace between the two boxes. The distance between the two boxes does not have to be equal all the way around. Also, keep in mind that it is very easy to adjust the size of a cardboard box by cutting and gluing it.
One sheet of cardboard to make the lid. This piece must be approximately 2 to 3 inch larger all the way around than the top of the finished cooker (the outer box). You'll also need a sheet of cardboard roughly the size of the perimeter of the smaller box.
One roll of aluminum foil.
One can of flat-black spray paint (look for the words "non-toxic when dry") or one small jar of black tempera paint.
At least 8 ounces (250 g) of white glue or wheat paste. For gluing the boxes and the pane of glass, I'd recommend extra strength paper cement.
One pane of simple glass. The pane should be about half an inch smaller in dimension than the outer box. The thicker the glass, the tougher it will be for solar rays to get in, but if the pane is too thin it will be easier to break. The pane that I use for my solar ovens is, on average, about .30 centimeters thick.
One sharp knife for the assembly process.
An 8 to 12-inch piece of stiff wire, like a wire coat hanger, that will serve to hold up the flap on the lid. The most important material we have is the cardboard boxes. The better shape the boxes are in, the hotter our solar oven will get. This means look for boxes without tears, rips, holes, etc. Larger ovens will get hotter faster, but keep in mind that it will be more difficult to find a solid piece of cardboard with which to make the top. It doesn't really matter how much bigger the outer box is as long as there is at least a half inch of difference between the two boxes around the sizes. Don't worry if the inner box is taller, we'll take care of that. I've found that one of the best places to look for boxes is places that they sell appliances like microwaves, refrigerators, and computers. When choosing the piece of cardboard to make the top, keep in mind that it'll be better if the piece has no bends or creases and is generally a thicker kind of cardboard. Like I said, the bigger your oven is, the harder this piece will be to find. Try finding a refrigerator box to cut this piece out of.
Make sure your paint is non-toxic when dry! The last thing we want is to make an awesome solar oven that we can?t use because the paint we used is toxic when dry. All you have to do is check the label. The pane of glass is another vital part of our solar oven. While a thinner pane will let rays pass more easily into the oven, it will also be more likely to break. The panes I?ve used on my ovens are generally about 1/8 of an inch thick. Make sure the pane doesn't have any kind of tinting or treatment that would bounce off our precious solar rays.
Lastly, your cutting tool is important. Out of necessity, I use my Leatherman multi-tool, but any sharp knife will do. The crafting tools like X-acto-knives with retractable blades usually work well. The most important thing is that it?s sharp and cuts through cardboard well, because we need crisp cuts. Now that we've got all our tools and materials together, let's get to building!
Fold the top flaps closed on the outer box and set the inner box on top and trace a line around it onto the top of the outer box.
Remove the inner box and cut along this line to form a hole in the top of the outer box. This step can be a bit easier if you tape the flaps on the outer box.
Now we need to decide how deep we want our oven to be. It should be about one inch deeper than our largest pot and about one inch shorter than the outer box so that there will be a space between the bottoms of the boxes once the cooker is assembled.
Using a knife, slit the corners of the inner box down to that height.
Fold each side down to form extended flaps. Folding is easier if you first draw a line from the end of one cut to the other where the folds are to go. Make sure that this fold is crisp and straight. It helps me to fold along the edge of a desk or table.
Next comes the aluminum foil. This is probably the most time-consuming of all the steps. Using the white glue, glue aluminum foil to the inside of both boxes (the walls and the bottom) and to the inside of the remaining flaps of the outside box.
Don't bother being neat on the outside box because it won't experience any wear and tear and won't be seen, either. On the other hand, you might want to take your time on the inside box because it will be visible. Now crumple up newspaper and place it at the bottom of the outside box. Put enough newspaper so that when you put the inside box in, the folds on the flap sit flush with the top of the outside box. The newspaper not only supports the box but also acts as insulation to prevent heat from escaping the oven, so the more the better. Open the flaps on the outer box to allow you put more newspaper in between the sides of the two boxes.
Once you're done, close the flaps, glue them, and then glue on top of those the flaps of the inner box. If the inner box sits too high and the flaps don't sit perfectly flush on the edge of the outer box, take out some of the newspaper in the bottom. Trim the excess flaps to be even with the perimeter of the outer box. One way to get a good seal between the two boxes is to flip them upside down and then put some kind of weight on top. If you really want to make sure no heat escapes, you can put aluminum foil all around the edge of the box.
Finally, to make our drip pan, cut a piece of cardboard that?s the size of the base of the inner box. Cover this piece with tin foil and paint the top black, in order to attract the solar rays. Place a few nickel-sized pieces of cardboard on the bottom of the oven and then rest the drip pan on them so that it sits slightly above the base of the oven. We're now done with the base!
Take the large sheet of cardboard and lay it on top of the base. Trace its outline and then cut and fold down the edges to form a lip of about 3" (7.5cm). Fold the corner flaps around and glue them to the side lid flaps. Because the glue often doesn't dry quickly enough, a good trick I found is to tape them, as well. Once the glue dries, you can remove the tape. Orient the corrugations so that they go from left to right as you face the oven so that later you can prop the wire into the corrugation. Don't glue this piece to the box! We're going to need to be able to take it on and off.
To make the reflector flap, draw a line on the lid, forming a rectangle the same size as the oven opening. Cut around three sides and fold the resulting flap up forming the reflector. Foil this flap on the inside. Make sure you don't cut all four sides because then we won't have a flap at all, just a hole.
Next, turn the lid upside down and glue the pane of glass into place. A trick I've found to ensure the glue seals as much as possible is to carefully put heavy books on top of the glass. By spreading the weight around evenly, we get a good seal between the cardboard and the glass.
The last step is to bend the wire into a "Z" shape that we can then use to prop open the flap. We're done!
The oven obviously works best when the sun is strongest. That is to say, the best cooking times are between 10 am and two pm. Just because there are clouds doesn't mean the oven won't work; as long as you have sun 60% of the time, your oven should cook. It also helps if you use a black pot or pan that has its own glass top. Because the design is all about storing up heat, don't take off the top unnecessarily because then you'll lose all the heat you've generated. Lastly, any hole or gap on the oven will mean less heat. Take your time to ensure that any possible place that heat can escape is insulated, glued, or has aluminum foil over it. The more careful you are during the construction process, chances are the hotter your oven will get.